Swelling Clays Effect to the Reservoir Well Testing, a Case Study from the Mesozoic Clastics Formation in South of Oman

Abstract

The Mesozoic Clastics Formation refers to a sequence of continental clastics encountered in South Oman Salt Basin that are potentially age is equivalent to the Kahmah Group of the Hajar Supergroup. Petroleum Development Oman’s (PDO) exploration campaign of the heavy oil opportunities targeted the Natih, Gharif and Mesozoic Clastics Formations in the Eastern Flank area of South Oman. A four well drilling campaign helped to provide insight of the reservoir and hydrocarbon distribution and confirmed that the Gharif and Mesozoic Clastics are the main pay zones. However, during an acid stimulation job of the reservoir zones in one well, it was noticed that the pre-stimulation injectivity and post-stimulation flowback were poorer in Mesozoic Clastics unit despite the similarity in fluid viscosity and mobility between the Gharif and Mesozoic reservoirs. A mineralogical analysis of Quantitative Evaluation of Materials by Scanning Electron microscopy (Qemscan), Scanning Electron Microscope (SEM) and X-Ray Diffraction (XRD) analysis were performed on core plugs in the Mesozoic Clastics reservoir unit to determine the reason behind the poor injectivity and well test results. The analysis showed that the clay minerals are common throughout the analyzed sections and typically occur in major quantities (10.8 to 49.9 mass % total clay). A diverse range of clay minerals are identified including non-ferroan illitic clays, kaolinite, smectite, chlorite and ferroan illitic clays. XRD analysis also showed a high amount (up to 7.2%) of the swelling clay smectite. These clay minerals occurs in different forms as matrix, thick cutan-like coatings or as grain-like masses. In addition, some of these clays are present as pore lining cement of booklets (kaolinite) or pervasive pore filling cement (smectite). A second workover of the well was arranged to test the Mesozoic Clastics unit with adjusted treatments such that smectite would not swell. The results were better than the first well test, suggesting that the presence of smectite was the main reason for the first well test failure. This project is a good example of the necessity of integration and collaboration between different disciplines to adapt best practices and decisions to tackle reservoir challenges and achieve a successful operation. The success of this campaign unlocked possibilities for similar discoveries and provided more understanding of the sedimentology of the Mesozoic Clastics Formation in the area.

AAPG Datapages/Search and Discovery Article #90360 © 2019 AAPG Middle East Region Geoscience Technology Workshop, 3rd Edition of the Siliciclastic Reservoirs of the Middle East, Muscat, Oman, October 28-30, 2019